Budget exhaustion is not enforced after cost metrics #25

Cost metrics decrement the budget ledger, but no code checks whether the counter reached zero afterward. BudgetLedger.AssertNotExhausted exists and the XML comment on Job.EmitMetricAsync says a BudgetExhaustedException may be raised, yet EmitMetricAsync only calls ApplyMetric and then emits the metric event. A job can keep running after spending through its declared cost.budget, while the docs and conformance page state that BUDGET_EXHAUSTED is enforced.

The relevant locations are src/Arcp.Runtime/Job.cs:151, src/Arcp.Runtime/Job.cs:155, src/Arcp.Runtime/Budget/BudgetLedger.cs:51, docs/guides/jobs.md:129, and docs/conformance.md:33.

Fix prompt: Enforce budget exhaustion immediately after applying a matching cost.* metric. Update Job.EmitMetricAsync to call BudgetLedger.AssertNotExhausted after ApplyMetric returns true, convert exhaustion into the protocol behavior described by the docs, and ensure the job terminates with ErrorCode.BudgetExhausted and Retryable = false rather than continuing. Add unit and integration tests for a lease with cost.budget of USD:1.00, a metric of cost.inference with value greater than or equal to the budget, and the expected BUDGET_EXHAUSTED terminal result.